<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      #app {
        position: relative;
        width: 100%;
        height: 800px;
        background-color: #999;
        border: 1px solid #000;
      }
      .inner {
        width: 100px;
        height: 100px;
        position: absolute;
        left: 0;
        right: 0;
        top: 0;
        bottom: 0;
        margin: auto;
        background-color: red;
      }
    </style>
  </head>
  <body>
    <div id="app">
      <div class="inner"></div>
    </div>
  </body>
</html>
